Self-healing capacitors

Mtiking a ctipacitor element A critictil review of internally protected capacitor units Self-healing capacitors Making a capacitor unit from elements Making capacitoi banks from eapticitor units Rating... 

Shunt power capacitors of the self healing type for a.c. systems having rated voltage up to and including 1000 V 13340/1993 BS EN 608.31 -1/1998... 

Shunt power capacitors of non self-healing types up to and including 1000 V General performance, testing and rating. Safety requirements. Guide for installation and operation 13585-1/1994 BS EN 60931-1/1996... 

Systems prone to frequent voltage variations Capacitor elements are non-self-healing and are generally provided with internal fuses An inductor coil may not be necessary with such capacitors... 

The metallized film capacitors have the characteristic of self-healing. On a small dielectric failure the capacitor element is not rendered completely unserviceable. After clearing the fault, the affected capacitor element returns to the circuit and the capacitor unit functions normally. Only the punctured area is eliminated from the element and causes a negligibly small reduction in its capacitance value. Such a characteristic is termed self-healing and such capacitors are known as the self-healing type. 

For LT capacitor units up to 1000 V For both self-healing and non-self-healing types ... 

Shunt capacitors for a.c. power system rated voltage above 1000 V (non-self-healing type) Endurance testing 13925-2/1994 BS 7264-2/1990... 

The use of metallized electrodes on certain polymers provides the capacitor with self-healing properties during manufacture and throughout its working life. If breakdown should occur at a fault in the dielectric layer, the local heating causes the deposited aluminium in the vicinity to evaporate, leaving an area bare of metal around the hole produced by the breakdown. 

Figure 1.52 Self-healing mechanism ofTa capacitors.

Traditionally, off-line X-caps were of special metallized film + paper construction whereas Y-caps were a specially constructed disc ceramic type. However we can also find X-caps that are ceramic, as we can find Y-caps that are film type. It s a choice dictated by cost, performance, and stability concerns. Film capacitors are known to always provide much better stability over temperature, voltage, time, and so on — than most ceramics. In addition, if they are of metallized construction, they also possess self-healing properties. Note that ceramic capacitors do not have any inherent self-healing property. However, ceramic Y-caps are specifically designed never to fail shorted under any condition, as this would pose a serious safety hazard. 

A. Yoshizawa, M. Takeda, Y. Oura, Y. Takemoto and K. Naoi, Low-molecular-weight soluble polyaniline for electrolytic capacitor, Electrochemistry, 1999, 67, 45 H. Yamamoto, K. Kanemoto, M. Oshima and I. Isa, Self-healing characteristics of solid electrolytic capacitor with polypyrrole electrolyte, Electrochemistry, 1999, 67, 855 M. Mastragostino, R. Paraventi and A. Zanelli, Supercapacitors based on composite polymer electrodes, J. Electrochem. Soc., 2000,147, 3167. 

In contrast, self healing properties have been attributed to PEDTtPSS cathodes in capacitors. As a potential mechanism, it has been postulated that high temperatures caused by leakage currents locally render PEDT PSS nonconductive [63], thus preventing further leakage and capacitor failure. 

Second type of self-healing process came about in tantalum capacitors with polypyrrole cathode (Harada 1997). This material thermally decomposes at about 300°C. When increased current is flowing through the defect region, heat is generated causing the decomposition of polypyrrole, which becomes less-conductive and finally to be insulating. 

Figure 5. Time dependence of voltage on capacitor during self-healing event for Mn02 cathode.

The main goal of heat conduction modelling was to determine, whether sufficiently high temperature could be reached during the capacitor internal discharge due to the breakdown to allow self-healing process in reasonable volume of tantalum capacitor. 

The most important sources of fluctuation in tan-taliun capacitors consist in regenerative micro breaks, fluctuation of polarization and mechanical strain. The frequency dependence of the noise spectral density in mHz region gives the information on slow irreversible processes of tantalum pentoxide crystallizations and oxide reduction. The self-healing process can improve... 

Harada, D. 1997. Self-healing mechanism of Neo-capacitor, NEC Research Development, 38, (3), pp. 301-305. 

Pavelka, J. et al. 2002. Noise and self-healing of tantalum capacitors, 22nd Capacitor and Resis-tor Technology Symposium, CARTS March 2002, pp. 25-29. 

Panasonic offers both polyester and metallised polyester film capacitors. The non-inductive, high stability, epoxy coated polyester types are also claimed to offer high volumetric efficiency and low losses. Additionally the metallised polyester film types are claimed to be self-healing. Their structure is composed of a number of discrete units to confine and isolate any dielectric breakdown. Panasonic also supplies miniaturised versions whose height and length dimensions are less than 8 mm with a corresponding thickness less than 5 mm. 

---